It is known to scrub flue gases from fossil-fuel-fired combustion, e.g. from fossil-fuel power plant boilers, with a scrubbing suspension containing in addition to water one or more alkaline-earth materials which can react with the sulfur oxides in the flue gases and with sulfur oxides which are formed in the scrubbing tower to produce calcium sulfate dihydrate.
In general, lime can be added to the scrubbing solution which can be recirculated from the sump of the scrubbing tower to the spray heads disposed above this sump so that the spray or scrubbing liquid passes in counterflow to the flue gases to be scrubbed.
The term "lime" as used herein is intended to refer to calcium oxide, calcium hydroxide and even calcium carbonate, since all react with the sulfur oxides to produce calcium sulfate which is in suspension in the scrubbing liquid in the sump and which can be precipitated from the scrubbing liquid or settled therefrom to enable recovery of the calcium sulfate.
In the recent applications Ser. No. 515,919 and Ser. No. 493,984 we, among others, have described processes in which oxygen in the form of air, oxygen-enriched air or technical purity oxygen, can be introduced into the sump of the scrubbing column at least in part to ensure an oxidation of sulfite to sulfate whereby the solids in the suspension which is produced from the scrubbing operation can be predominately calcium sulfate dihydrate. The calcium sulfate dihydrate which is subsequently recovered from the sump suspension is a valuable product since it can be converted into a building material and frequently can be recovered as a usable gypsum product directly so that it need not be disposed of and thus does not create an environmental hazard. In this respect, reference may be had to Ser. No. 546,486 filed Oct. 28, 1983 now U.S. Pat. No. 4,503,020.
In this later application a process is described in which the sump liquid, i.e. the suspension recovered by scrubbing the gas, is recirculated from the sump to the spray heads. This ensures a predetermined mean residence time for the scrubbing liquid in the column even though scrubbing liquid from the column is continuously withdrawn from the cycle for further processing, e.g. for the recovery of the gypsum product.
The scrubber is designed generally for the maximum expected flue gas volume and maximum expected sulfur dioxide concentration in the flue gas.
In practice, while the scrubbing system is designed to be able to deal with these maxima, the sulfur content of the coal is usually less than the maximum permissible sulfur content and the power demand in the plant is generally less than that which would give rise to the maximum flue gas volume so that normal operations take place at levels well below these maxima.
Furthermore, the fluctuations in the flue gas volume and sulfur content are substantial, the quantities of sulfur dioxide which must be removed vary sharply and naturally the amount of calcium sulfate dihydrate which is recovered in the scrubbing liquid varies sharply with time.
It has been a practice in the past to maintain the concentration of the liquids, namely, the calcium sulfate dihydrate, in the recirculated scrubbing liquid substantially constant and to withdraw this scrubbing liquid substantially continuously for processing, i.e. gypsum recovery.
This, however, requires continuous control of the withdrawal pump and, in turn, requires the use of an expensive control circuit, operating with great precision and a variable displacement pump, e.g. a pump whose speed could be controlled and which generally was of the displacement type. Such pumps were subjected to considerable wear.
Furthermore, the downstream apparatus, i.e. the means for processing the suspension, had to be compensatorily designed to accommodate the carefully controlled withdrawal of the suspension from the cycle and thus the further processing of the suspension to yield the calcium sulfate dihydrate was required to match the variable rate of continuous outflow from the cycle. This resulted in variations in the quality of the calcium sulfate dihydrate or gypsum which was recovered and frequently gave rise to a poor quality product.